The demand for electricity continues to grow globally. In order to keep stride with the growing demand, coal continues to be a primary source for electricity generation. The burning of coal in power generation plants results in the release of energy, as well as the production of solid waste such as bottom and fly ash, and flue gas emissions into the environment. Emissions Standards, as articulated in The Clean Air Act Amendments of 1990 as established by the U.S. Environmental Protection Agency (EPA), requires the assessment of hazardous air pollutants from utility power plants.
Conventional coal-fired combustion furnaces and similar devices produce emissions that include pollutants such as mercury. Mercury vapor can contribute to health concerns. At the levels common in the atmosphere, the concentrations of mercury are usually safe. However, mercury can accumulate in ecosystems, for example, as a result of rainfall. Some conventional systems attempt to control mercury emissions with particulate collection devices.
The primary gas emissions are criteria pollutants (e.g., sulfur dioxide, nitrogen dioxides, particulate material, and carbon monoxide). Secondary emissions depend on the type of coal or fuel being combusted but include as examples mercury, selenium, arsenic, and boron. Coal-fired utility boilers are known to be a major source of anthropogenic mercury emissions in the United States. In December of 2000, the EPA announced its intention to regulate mercury emissions from coal-fired utility boilers despite the fact that a proven best available technology (BAT) did not exist to capture or control the levels of mercury released by the combustion of coal. This has been further complicated by the lack of quick, reliable, continuous monitoring methods for mercury.
Mercury (elemental symbol Hg) is a metal that melts at 234K (−38° F.) and boils at 630K (674° F.). As such, it can be expected to have a high vapor pressure relative to many metals. The oxidized forms of mercury, Hg2+ and Hg+, have much lower vapor pressures and can be captured by fly ash particulates.
Mercury is found in coals at concentrations ranging from 0.02 to 1 ppm. The mercury is present as sulfides or is associated with organic matter. Upon combustion the mercury is released and emitted into the flue gas as gaseous elemental mercury and other mercury compounds. The mercury appears in the flue gas in both the solid and gas phases (particulate-bound mercury and vapor-phase mercury, respectively). The so-called solid-phase mercury is really vapor-phase mercury adsorbed onto the surface of ash and/or carbon particles. The solid-phase mercury can be captured by existing particle control devices (PCDs) such as electrostatic precipitators (ESPs) and fabric filters (FF), the latter sometimes referred to as baghouses.
Several control strategies have been developed for the control of mercury emissions from coal-fired boilers. Some of these methods include injection of activated carbon, modified activated carbon, various chemical catalysts, and inorganic sorbents. Unfortunately, none of these strategies removes all the mercury from the flue gas. The efficiencies range from as low as 30% to as high as 80% based on the amount of mercury entering the system with the coal. In addition, these technologies either produce unwanted effects on by-products such as impacting the quality of fly ash, or they generate additional waste streams for the power plant. Both lead to higher operational costs for the power plant. One promising strategy is to take advantage of existing air pollution control devices (APCDs) to augment or to serve as the primary means to remove vapor-phase mercury. Two examples of APCDs are semi-dry and wet scrubbers or flue gas desulfurizer (FGD). Semi-dry FGDs are also known as spray dryer absorbers (i.e., SDAs), circulating dry scrubbers (CDS), or TURBBOSORP® available from Von Roll.
Sulfur oxides (SOx) regulatory compliance mandates the use of at least one of several control strategies. Three such strategies that are used in the US are sorbent injection into the flue gas following by a particulate collection device such as an ESP or a FF, and wet or dry flue gas desulfurizers. At present, about 3% of the coal-fired power plants are using sorbent injection. FGD scrubbing accounts for 85% using wet and 12% using dry scrubber technologies. Wet scrubbers achieve greater than 90% SOx removal efficiency compared to 80% by dry scrubbing. In wet scrubbers, the flue gas is brought into contact with slurry containing an alkaline source such as lime or limestone. The SOx is adsorbed into the water and reacts to form calcium sulfite. It has been demonstrated that simultaneous to SOx capture, wet FGDs can be used to capture vapor-phase mercury from the flue gas.
Elemental mercury is water-insoluble and is not removed by a wet FGD. In contrast, oxidized mercury in the flue gas is water-soluble and is removed. The Information Collection Request (ICR) mercury data demonstrated that ionic mercury is removed effectively approaching 90% by wet FGDs. Hence, one strategy for mercury capture is to oxidize all the mercury during the burning of the coal and capture the oxidized mercury in the wet scrubber. Work carried out by URS in conjunction with the Department of Energy/National Energy Technology Laboratory (DOE/NETL) investigated just such a strategy. There are two critical technical steps to the implementation of this strategy. The first is the complete oxidation of the vapor-phase mercury exiting the boiler and the coal. URS, among others, is developing strategies and technologies to accomplish this step. To date, they have demonstrated that independent of the coal type, vapor-phase mercury speciation can be shifted to extensively 100% oxidized mercury. The second critical technical step in the implementation of this control strategy is the sorption of the oxidized mercury and removal in the wet scrubber. The problem, identified early on, is that there are reactions occurring in the wet scrubber liquor that reduce oxidized mercury to elemental mercury and lead to “re-emission” or release of elemental mercury into the scrubbed flue gas. The prevention of ionic mercury reduction in wet scrubber liquor has been studied and reported by G. M. Blythe and D. W. DeBerry at URS and others.
The findings have suggested that complexation of the ionic mercury is one way to reduce or eliminate the generation of elemental mercury in the scrubber. This same study has demonstrated that not all chelants of ionic mercury can accomplish this in a wet FGD. In a recent presentation, plant results of such a chelant, TMT-15, trimercapto-s-triazine, available from Degussa, were inconclusive regarding the prevention of re-emission of mercury across a wet scrubber. Efficient and cost-effective apparatuses and methods for controlling emissions of mercury remain a desirable need in the art whether from combustion sources such as coal plants and cement kilns or other sources such as incinerators used in a variety of activities.